Oral bacteria have been implicated in the development of chronic periodontitis. These organisms may initiate periodontal inflammation by producing antigens which activate the cellular immune responses of their host. The specific nature of the antigens produced by the bacteria is not yet known. Studies are in progress to isolate and characterize the antigens from oral Actinomyces that activate cellular immune responses. In addition, the cellular and molecular events accompanying lymphocyte transformation by such oral bacterial antigens are under investigation. These studies may improve our understanding of the biochemical and immunological events which lead to chronic inflammation and destruction of periodontal tissues.